This invention relates to a high toughness (tenacious) cermet suitable as a material for cutting tools such as lathe cutting tools, slicing tools, drills and end mills, or a material for abrasion resistant and corrosion resistant tools such as slitters, cutting blades, dies for can making and nozzles, most suitable as a material for cutting tools, particularly as a material for wet cutting tools which require thermal shock resistance, and a process for preparing the same.
In the prior art, TiC-based cermets can be roughly classified into N (nitrogen)-non-containing TiC-based cermets and N-containing TiC-based cermets. Of these, N-containing TiC-based cermets tend to be more excellent in strength and plastic deformation resistance as compared with N-non-containing TiC-based cermets, For this reason, TiC-based cermets in recent days tend to be mainly N-containing TiC-based cermets.
However, N-containing TiC-based cermets have a problem that the surface portion of a sintered alloy is liable to be brottle (or fragile) as compared with the inner portion due to denitrification and carburization in a sintering step.
To cope with such a problem, a proposal of providing a surface portion preferred from the points of characteristics of a sintered alloy has been made, which is represented by Japanese Unexamined Patent Publications No. 31949/1989 and No. 15139/1990.
Japanese Unexamined Patent Publication No. 31949/1989 discloses a high toughness cermet obtained by imparting a compressive stress of 50 kg/mm.sup.2 or more to a hard phase at the surface portion of a burnt surface of a sintered alloy comprising a hard phase comprising at least one of carbide, nitride, carbonitride, oxynitride and boride of the 4a, 5a or 6a group metals of the periodic table and solid solutions of these, a binder phase composed mainly of Ni and/or Co, and inevitable impurities.
The high toughness sintered alloy disclosed in the above patent publication is an alloy improved in flexural strength and fracture resistance by imparting compressive stress thereto by applying impact force to the surface portion of the burnt surface by means of shot peening or sand blast. However, there involve problems that abrasion resistance and thermal shock resistance have not been taken into consideration, and particularly when it is used as a material for wet cutting tools, abrasion resistance is poor and also reliability of preventing sudden fracture caused by occurrence and progress of thermal cracking is poor.
Japanese Unexamined Patent Publication No. 15139/1990 discloses an N-containing TiC-based cermet having a maximum surface roughness of a burnt surface of 3.5 .mu.m or less, substantially free from pore and void, and having a hard and high toughness region provided at a surface portion. The cermet disclosed in the above patent publication is a cermet improved in abrasion resistance and fracture resistance by imparting high toughness and high hardness thereto by using a sintered alloy having high surface precision of a surface to be heated and substantially free from pore and void. However, there involve problems that fracture resistance is not satisfactory, thermal shock resistance is poor, and particularly when it is used as a material for wet cutting tools, reliability of preventing sudden fracture caused by occurrence and progress of thermal cracking is poor.